1. Field of the Invention
The present invention relates to a transmission control apparatus that controls the transmission mounted in a vehicle, such as an automobile, and, more particularly, to a transmission control apparatus that controls the transmission which separates power from an engine to two power transmission paths each having an independent clutch and an independent transmission mechanism.
2. Description of the Related Art
There has been a so-called twin-clutch transmission which separates power from an engine to two power transmission paths each having an independent clutch and an independent transmission mechanism, and performs a transmission operation to transmit power from the input shaft to the output shaft. This twin-clutch transmission uses an electric motor as an actuator to actuate the clutch (see, for example, Non-patent Document 1 (“Electromotoric actuators for double clutch transmissions” which is the document distributed in “8th Luk Symposium 2006”) held by Luk GmbH & Co., Germany)).
There is known a twin-clutch transmission control apparatus which controls a twin-clutch transmission so that when one of the transmission groups fails, the other transmission group is used to carry out the transmission operation (see, for example, Patent Document 1 (JP-A-2006-132574).
The twin-clutch transmission disclosed in the Non-patent Document 1 uses an electric motor as a clutch actuator, and uses a normal-open clutch which transmits power from the input shaft to the output shaft at the time the electric motor is energized (operated), and does not transmit power from the input shaft to the output shaft at the time the electric motor is not energized (not operated). When the electric motor is energized to transmit the power from the input shaft, a current corresponding to the supply voltage flows. A torque is generated in the electric motor according to the size of the current which has flowed in the electric motor. The greater the generated torque is, the greater the torque to be transmitted to the output shaft of the clutch from the input shaft thereof.
An electric motor generally consumes part of the electric energy as heat, so that as the energization time gets longer, the quantity of energy to be given to the electric motor becomes larger and the temperature of the electric motor rises to the temperature at which heat balance with the ambient temperature is established.
The size of the torque of the electric motor is determined by the magnetic flux that is generated by the current having flowed across the coil in the electric motor, and the magnetic flux that is generated by the permanent magnet disposed near the coil, and the permanent magnet has a property such that as the temperature of the permanent magnet rises, the size of the magnetic flux that is generated becomes smaller. As the energization of the electric motor raises the temperature of the coil, therefore, the temperature of the permanent magnet rises, so that even with the same electric energy supplied to the electric motor, the temperature of the permanent magnet rises, thus making the size of the torque generated in the electric motor smaller. As a result, the torque generated in the electric motor becomes smaller than the torque needed to hold the clutch, so that when the electric motor becomes a performance-dropped state, the power transmission performance of the clutch becomes lower.
When the power transmission performance of the clutch becomes lower, clutch slipping occurs, causing friction to generate heat at the clutch portion. The heat warms up near the clutch, further increasing the temperature of the nearby electric motor. The coil of the electric motor is made of a metal, and has its surface coated with an insulator. When the internal temperature of the coil becomes a failure limit temperature or higher, the high temperature causes a chemical reaction of the coat of the insulator, which damages the insulation of the conductive wire or disconnects the soldered portion of the coil, resulting in a failure of the electric motor.
As described above, when the performance-dropped state of the electric motor continues and the motor temperature rises, a failure, such as line disconnection or short-circuiting, is likely to occur. To prevent a failure, therefore, it is necessary to prevent the performance-dropped state of the electric motor from continuing for a long period and prevent the temperature of the electric motor from becoming equal to or higher than the failure limit temperature.
When a failure in one of electric motors used in the twin-clutch transmission cannot be prevented and one electric motor fails, this electric motor cannot generate a torque, so that the clutch which is operated by the electric motor cannot transmit power from the input shaft to the output shaft. Accordingly, gear shifting of the transmission is carried out only with the clutch that includes the other electric motor, so that the gear ratio of the transmission cannot be set to one which ensures a high fuel consumption effect. Because the adequate gear ratio cannot be selected, the acceleration intended by a driver may not be actuated or the high-speed driving may make the engine sound louder, which is the deterioration of the drivability. Further, the gear shifting operation at a gear ratio far different from the normal one would bring about a problem of a continuous state where, for example, the gear shifting shock would become greater.
In addition, after instructing the operation of the clutch actuator, the transmission control apparatus disclosed in the Patent Document 1 detects if a clutch actuator has actually been operated through stroke detection or the like to determine the occurrence of a failure. The transmission control apparatus disclosed in the Patent Document 1 has problems such that although the transmission control apparatus can detect a failure after the clutch actuator has failed, it cannot detect that the clutch actuator is in a performance-dropped state while the driving of the clutch actuator is being instructed, and thus keeps operating until the clutch actuator fails, so that the failure cannot be prevented.